Question

scala> val set = scala.collection.mutable.Set[(Int, Int)]()
set: scala.collection.mutable.Set[(Int, Int)] = Set()

scala> set += (3, 4)
<console>:9: error: type mismatch;
 found   : Int(3)
 required: (Int, Int)
              set += (3, 4)
                  ^

scala> set += Tuple2(3, 4)
res5: set.type = Set((3,4))

Adding (3, 4) does not work – why ?

Normally, (3, 4) also represents a tuple with two elements.

Answer 1

The issue is that it exists in the Set trait a method +(elem1: A, elem2: A, elems: A+) and the compiler is confused by it. It actually believes that you try to use this method with 2 Int parameters instead of using it with a tuple, as expected.

You can use instead: set += (3 -> 4) or set += ((3, 4))